It is known that when a groove is formed in a processing target film in an etching process using plasma, so-called bowing in which a sidewall of the groove is etched excessively and a space in a middle portion of the groove is swollen occurs. The bowing problem becomes obvious as miniaturization progresses and it is difficult to control a width of the groove to a designed level. To that end, Japanese Patent Application Publication No. 2010-109373 discloses a technique for adding COS (carbonyl sulfate) gas to an etching gas such as O2 (oxygen) gas, N2 (nitrogen) gas, H2 (hydrogen) gas or the like.
By adding COS gas to the etching gas, the bowing is suppressed to a certain extent. However, as the miniaturization progresses further, the bowing needs to be suppressed further.